Hmmmm.

The two reactors at the San Onofre nuclear-power station near San Clemente, Calif., will remain shut down this weekend while federal safety officials investigate why critical—and relatively new—equipment is showing signs of premature wear.

The problem surfaced Jan. 31, when one of the units sprang a leak in a pipe called a steam tube, releasing small amounts of radioactive steam and tripping radiation alarms. Operators shut down the reactor four hours later.

The problem rests in the steam generators in these plants (they're pressurized water reactors.)  These are sort of like a car radiator, except that both sides are in water (if you have a boat with an inboard engine you've probably got a heat exchanger that is much like this in the cooling system.)  The steam generator has primary water under very high pressure that takes the heat off the nuclear reaction and uses it to boil water in the secondary side, which turn turns the generating turbines.  The advantage of this design is that the water on the primary side never boils and it is physically isolated from the steam turbine equipment.  Since the primary side water becomes activated over time (that is, it is somewhat radioactive) this prevents the turbine equipment from being contaminated -- at least in theory.  A boiling-water reactor (such as in Fukushima) on the other hand has only one loop; there are advantages and disadvantages to both designs.

Unfortunately like a car radiator these tubes wear out and can leak.  Due to the high pressures and temperatures involved the tubes are made out of a special alloy. 

These reactors were shut down a couple of years ago after the original steam generators had deteriorated to the point that they needed replacement -- they were nearly 30 years old at the time and had reached the end of their service life.  The replacements were made out of a somewhat different alloy that was intended to address cracking problems that had intermittently shown up in the old design.

But now, with only two years of runtime on the new generators, some of the tubes are 10-20% eroded and a few had reached 35%.  This is important because the usual engineering standards allow for 30% erosion at which point the tubes must be taken out of service, and when about 10% or so of the total have been removed from service the entire generator has to be removed and either reworked or replaced.  This is a rate of deterioration that is several times that of the original, more than 5x as fast as expected!

Mitsubishi Heavy made the new replacements -- the old ones were made here in the United States.  Of course Mitsubishi Heavy is a Japanese company, but one wonders what the original source of the materials that went into that tube bundle actually was.

So here's the question: Was the choice of alloy wrong -- an engineering failure -- or was there a manufacturing and inspection problem responsible? 

And if the latter, where did the materials come from that the tubes were fabricated out of, and exactly where did that fabrication take place?

I suspect that I'm not going to like the answers to these questions.